Three months after the First Battle of Bull Run, Major General George B. McClellan was building up the Army of the Potomac in preparation for an eventual advance into Virginia. On October 19, 1861, McClellan ordered Brigadier General George A. McCall to march his division to Dranesville, Virginia, twelve miles southeast of Leesburg, in order to discover the purpose of recent Confederate troop movements which indicated that Colonel Nathan "Shanks" Evans might have abandoned Leesburg. Evans had, in fact, left the town on October 16–17 but had done so on his own authority. When Confederate Brigadier General P.G.T. Beauregard expressed his displeasure at this move, Evans returned. By the evening of October 19, he had taken up a defensive position on the Alexandria-to-Winchester Turnpike (modern-day State Route 7) east of town.
McClellan came to Dranesville to consult with McCall that same evening and ordered McCall to return to his main camp at Langley, Virginia, the following morning. However, McCall requested additional time to complete some mapping of the roads in the area and, as a result, did not actually leave for Langley until the morning of October 21, just as the fighting at Ball's Bluff was heating up.
On October 20, while McCall was completing his mapping, McClellan ordered Brigadier General Charles Pomeroy Stone to conduct what he called "a slight demonstration" in order to see how the Confederates might react. Stone moved troops to the river at Edwards Ferry, positioned other forces along the river, had his artillery fire into suspected Confederate positions, and briefly crossed about a hundred men of the 1st Minnesota to the Virginia shore just before dusk. Having gotten no reaction from Colonel Evans with all of this activity, Stone recalled his troops to their camps and the "slight demonstration" came to an end.
Stone then ordered Colonel Charles Devens of the 15th Massachusetts Infantry, stationed on Harrison's Island, facing Ball's Bluff, to send a patrol across the river at that point to gather what information it could about enemy deployments. Devens sent Captain Chase Philbrick and approximately 20 men to carry out Stone's order. Advancing in the dark nearly a mile inland from the bluff, the inexperienced Philbrick mistook a row of trees for the tents of a Confederate camp and, without verifying what he saw, returned and reported the existence of a camp. Stone immediately ordered Devens to cross some 300 men and, as soon as it was light enough to see, attack the camp and, per his orders, "return to your present position."
This was the genesis of the Battle of Ball's Bluff. Contrary to the long-held traditional interpretation, it did not come from a plan by either McClellan or Stone to take Leesburg. The initial crossing of troops was a small reconnaissance. That was followed by what was intended to be a raiding party. To make matters worse, Stone was not advised that McCall and his division had been ordered back to Washington.